How Do Cities Treat Drinking Water? (Municipal Water Treatment Explained)

Imagine living in a city without a water treatment system. As the water journeys from its source to your home, it could flow through zinc, sulfur, and arsenic-laden formations, bringing toxic chemical elements into your drinking glass. Groundwater could pick up organic minerals, industrial chemicals, heavy metals, and pollutants from fertilizers, septic tanks, and mine drainage, contaminating your drinking water. Rivers and streams could carry harmful micro-organisms from animal and human waste, and storm drains could wash runoff from cities into rivers and streams, which could escape into your home’s water supply. And the likely result? Smelly, nasty-looking water gushing out of your taps and entering your water heater, washing machine, and dishwasher!

Thankfully, U.S. municipal drinking water supplies are heavily regulated. Public water utilities operate rigorous water treatment systems that use various technologies and processes to remove anything in the water that could endanger our health. They also must ensure the water meets the standards set by the Environmental Protection Agency (EPA).

City water treatment is crucial to why we have clean, drinkable water in our homes. But how does it work? How exactly does a treatment plant take dirty water from a river and turn it into clean water?

What is Municipal Water?

Municipal water, also known as city water, is water distributed through a public supply network owned or maintained by the city. Usually, this water goes through a complex filtration and purification process to ensure it is clean and healthy enough to be delivered to rural, suburban, and urban populations.

While a significant amount of municipal water is sent to homes after being treated, enormous quantities are transported to local businesses and industrial plants to be used in a wide range of manufacturing processes.

The main uses of treated municipal water in homes include drinking, cooking, bathing/showering, cleaning, washing, and watering lawns and gardens.

Why Do We Need Water Treatment?

Municipal water treatment is far more vital than most people think – and we’ll explain why. See, tap water in cities and other urban areas comes from surface water and groundwater, which can quickly become contaminated. Because of this tremendous contamination risk, water from these sources requires proper treatment before being dispersed to our homes.

Surface water sources include rivers, lakes, streams, creeks, wetlands, reservoirs, and other water bodies that collect on the ground’s surface. Because these surface sources are above ground, they are more open to contamination via pollution. For example, rainwater runoff from farms and crop fields can wash nitrates, pesticides, and animal manure into rivers and waterways, which can find their way into the local drinking water supply. Surface water typically requires more extensive treatment than groundwater because it often contains more suspended silt, organic materials, decaying vegetation, and microbes from animal waste.

Groundwater is typically found in sand, gravel, and bedrock pore spaces beneath the soil’s surface. It is the most common source of drinking water in the U.S. But like surface water, groundwater also needs treatment to make it drinkable. Examples of groundwater sources include aquifers, underground springs, and lakes. Water from these sources is typically free of microbes and suspended solids because of natural filtration as the water moves through rock and soil. However, groundwater often contains relatively high concentrations of dissolved minerals, organic contaminants, and heavy metals like arsenic picked up through seepage and direct contact with rock and soil. Pollutants and impurities in surface water can also leach into groundwater.

Because of the risk of surface- and groundwater contamination, the EPA has established limits for various contaminants in water, meaning the water must meet specific standards before reaching public water lines. As a result, municipalities must first treat and test the water to ensure it meets national or state standards.

The entire process of city water treatment helps reduce the levels of many contaminants affecting municipal water. These contaminants include:

  • Micro-organisms (cryptosporidium, giardia lamblia, legionella, fecal coliform, E. coli, etc.)
  • Disinfectants (chlorine, chloramines, chlorine dioxide, etc.)
  • Disinfection Byproducts (haloacetic acids, total trihalomethanes, chlorite, bromate, etc.)
  • Inorganic Chemicals (arsenic, asbestos, barium, beryllium, cadmium, chromium, fluoride, lead, mercury, nitrate, nitrite, etc.)
  • Organic Chemicals (atrazine, benzene, 2,4-D, glyphosate, polychlorinated biphenyls, etc.)
  • Radionuclides (uranium, radium, alpha particles, beta particles, photon emitters, etc.)

If there are too many contaminants in the water when you drink it, or it contains unsafe levels of certain pollutants, you could be at a higher risk of developing health conditions, such as:

  • Neurological disorders
  • Cardiovascular conditions
  • Reproductive issues
  • Gastrointestinal problems

Older adults, pregnant women, younger children, infants, and individuals with weak immune systems are particularly susceptible to these health problems. The good thing is that most American cities use advanced treatment systems to filter and purify millions – if not billions – of gallons of water daily. Treating the water helps ensure the concentration of contaminants in the water is at a “safe” level. Otherwise, people in the local community could become sick after drinking the water or ingesting foods and beverages made with the contaminated water.

The City Water Treatment Process

The main goal of water treatment is to protect public health. That means ensuring the public water supply is clear, with almost no turbidity, free of objectionable color, odor, taste, harmful micro-organisms and chemicals, and aesthetically desirable. But few of us know how municipalities treat water and how it gets to our homes.

The first thing to know is that municipal water treatment methods can vary from city to city, depending on the plant’s technology and the quality of the untreated water. For example, the water treatment system in a community relying on surface water may be more advanced than one in a town a few miles away, whose primary water source is groundwater. The better the quality, the less treatment is needed. However, most cities take a similar approach to water treatment.

The following describes the standard water treatment process in most U.S. towns and cities.

1. Collection

All drinking water starts at the source (for example, a freshwater river, lake, stream, or underground reservoir). Water utilities must find a way to get the untreated water to the water treatment plant, so they often use different extraction methods based on the source.

In the case of surface water, utilities often use a series of pumps and large pipelines to extract the raw water from the source and transport it to the water treatment plant.

The extraction procedure is slightly different for groundwater. Firstly, large holes are dug or bored in the ground to access aquifers and other underground sources containing groundwater. Pumps driven by electric motors raise the water from the aquifer to the surface. Finally, the extracted water is loaded into large trucks and transported to the water treatment facility to be filtered and purified.

2. Screening

If you’ve ever gone hiking or jogging and had to drink water from a river or other surface water source, you’ve probably realized it had a weird taste. That’s because surface water typically contains varying amounts of suspended or dissolved materials, including sediment, dirt, debris, sand, silt, etc. Larger and chunkier impurities, like leaves, fish, twigs, paper, rags, and other debris, could obstruct flow through the plant, damage equipment, and reduce efficiency. For this reason, the utility uses screens to remove any large floating and suspended solids in the inflow.

This screening process involves using a large metal coarse screen to stop large objects from entering the pipes. Fine screens are then used to keep out materials that could block pipes at the treatment plant. These fine screens can trap suspended matter as small as algae and plankton (microscopic organisms that float with the current in water). The trapped solids are removed from the screen fabric by high-pressure waterjets using clean water and carried away for disposal.

3. Chemical Coagulation

At this point, the drinking water has arrived at the water treatment plant, with all large debris removed. However, plenty of fine particles and debris are still saturating the water. These materials are too small to filter through a screen or settle, so a chemical coagulant must be added to the water to clump them together.

Once the coagulant is added to the water supply, high-speed mixers are used to distribute it evenly. Chemical coagulants, such as aluminum sulfate (alum), ferric sulfate, and sodium aluminate have a positive charge that neutralizes the negative charge of the suspended and dissolved particles. This reaction causes the particles to bind together and settle at the bottom of the water in thick clusters called flocs.

4. Flocculation

Once coagulation is complete, the mixers must slow down to allow the flocs to enlarge and prevent them from separating again. This gentle agitation or slow mixing is called flocculation. During this process, a polymer is added to the water to bind smaller flocs into larger masses. This ensures the flocs don’t break apart when the water is agitated. It also makes the clusters easier to remove. After flocculation is complete, the large solid masses can be removed from the water stream.

5. Sedimentation and Clarification

After the flocculation process is finished, the mixture of water and flocs goes to a sedimentation basin called a clarifier. The purpose of sedimentation is to remove suspended solids that are heavier than the water and reduce the particulate load on the filters. The floc is heavy, so it settles to the bottom of the tank in removable layers. The water is kept in the tank for several hours for sedimentation to complete. The longer the water sits undisturbed, the more solids will succumb to gravity and fall to the container floor.

The material that accumulates at the bottom of the tank is called sludge; this is removed for disposal. However, because sedimentation mainly removes larger particles, some smaller particles may remain, as well as bacteria, viruses, and unwanted chemicals. This brings us to the next step: filtration.

6. Filtration

Even after sedimentation, impurities that weren’t separated in the sedimentation tank may still be present in the water. Therefore, the water must be filtered to remove solid contaminants, such as sediment not previously removed, bacteria, viruses, and unwanted chemicals.

Filtration helps remove these impurities by passing the water downward through beds of porous, granular materials such as sand, gravel, and charcoal. Suspended particles become trapped within the pore spaces of the filter media. Filters with a small enough pore size may be able to remove bacteria, viruses, parasites, and protozoa.

When the filters are saturated with trapped solids, they are backwashed. During the backwashing process, clean water and air are pumped back onto the filter to dislodge the trapped impurities. The water carrying the dirt (referred to as the backwash) is pumped into the sewerage system if there is one. Alternatively, it may be discharged back into the source river after a settlement stage in a sedimentation tank to remove solids.

Water treatment plants that use recycled water from sewage, stormwater runoff, or industrial waste may need more advanced filtration technologies, like reverse osmosis or carbon filtration. Reverse osmosis removes most contaminants found in recycled water. However, many conventional plants are now using granular activated carbon as the media of choice because it provides excellent mechanical filtration of particulate matter and removes organic compounds, which can cause taste and odor problems.

7. Disinfection

Now that the water is filtered, you might think it is safe to drink. Not yet, but it’s getting there. The utility must disinfect the water to eliminate any remaining pathogenic micro-organisms.

The most used chemical disinfectant is chlorine, a liquid (such as sodium hypochlorite, NaOCl), or a gas. Chlorine is relatively cheap, easily accessible, and simple to use. When added to water, the chemical reacts with any pollutants present, including micro-organisms, over a given period. Chlorine kills pathogens such as bacteria and viruses by breaking the chemical bonds in their molecules. Tiny amounts of the chemical remain in the water as it flows to your home, protecting the water from any micro-organisms that might get picked up along the way.

Some municipalities use other disinfection methods, like ultraviolet (UV) radiation and ozonation. UV is highly effective at destroying pathogens but can be costly to implement on a large scale. It is also a one-time treatment option and doesn’t protect the water after it leaves the plant. Ozonation is also an effective water disinfection method, but since ozone is unstable, it cannot be stored and must be produced on-site, making it more expensive than chlorination.

8. Additional Treatment

In some cases, the water might need added treatment to benefit a population. One such instance is adding sodium fluoride or other fluorine compounds to filtered water to reduce incidences of tooth decay in young children. Some municipalities may include aeration in their water treatment process. Aeration is a physical treatment process that expels soluble gases such as carbon dioxide and hydrogen sulfide. These gases are acidic, so aeration makes the water less corrosive and expels any gaseous organic compounds with an undesirable taste to the water. Aeration may also remove iron and manganese known to stain clothing and cause peculiar tastes in water.

9. Storage

Once the disinfection process is complete and supplementary treatment is done, the water is stored. The water is usually held in an underground storage tank called a “clear well” and in elevated water towers visible around town. Water utilities must keep ample amounts of treated water in safe storage as preparation for emergencies, such as fires, floods, and power outages.

10. Distribution

The final stage of the overall process is getting the water to your home. The stored water is forced under pressure through large underground pipes all over town. This network of large and small pipelines, water mains, water pumps, fire hydrants, storage tanks, water meters, etc., is called a distribution system. The municipality uses this network to distribute the treated water to homes, businesses, farms, and factories.

Is City Tap Water Safe to Drink?

City-treated water often undergoes rigorous treatment and extensive monitoring to ensure it is healthy and safe to drink. However, the 2014 Flint, Michigan water crisis has taught us never to take anything for granted regarding tap water quality.

The Flint crisis occurred after the city switched its primary water source to the Flint River. The water treatment systems were ill-equipped to treat a highly corrosive water supply. The Flint River water was so corrosive that it caused heavy metals to leach from the city’s outdated lead pipes into the water supply after the water had already passed through the treatment facilities. Lead contaminated the drinking water supplies, exposing the local population to the metal and causing many health issues. In addition, the water was inadequately disinfected with chlorine, causing a spike in disease-causing legionella bacteria. So, the bottom line is that even if your drinking water is thoroughly treated at your local plant, there’s still the risk of contamination along the water system.

But doesn’t The EPA regulate how much of each contaminant is allowed in city-treated water, and aren’t municipalities held accountable if the water does not meet EPA standards? Great questions – and the answer to both is yes. But keep in mind that the EPA only regulates amounts unlikely to cause health effects in the long term.

There are ideal levels of contamination in drinking water and realistic levels a water treatment plant can achieve. For example, the ideal lead concentration in water is zero, but eliminating lead from water would significantly increase treatment costs. Because of this, the EPA mandates that lead levels must not exceed 15 parts per billion (ppb) in treated water.

But here’s the problem with that guideline: No amount of lead in drinking water is safe. Hence, lead concentrations below EPA levels can have severe side effects, including decreased kidney function, reproductive harm, increased blood pressure, reduced intelligence, and behavioral problems in children.

Ensuring Your Tap Water is Entirely Safe

Although city water is usually treated to remove a broad range of contaminants, investing in a home filtration system will decrease the chances of adverse health effects down the road. A reverse osmosis system is your best bet if you wish to make your drinking water as pure as possible.

With a pore size as small as 0.0001 microns, a reverse osmosis system is a highly effective solution against a substantial number of contaminants in water, including:

  • Metals and metalloids (for example, lead, chromium, mercury, iron, copper, aluminum, arsenic, etc.)
  • Chemical pollutants (such as PFAS, BPA, chlorine, chloramine, chlorine byproducts, chloride, pesticides, herbicides, fluoride, sodium, potassium, phosphorous, nitrate, etc.)
  • Bacteria (including, Campylobacter, Salmonella, Shigella, and E. coli)
  • Viruses (for example, Enteric, Hepatitis A, Norovirus, Rotavirus)
  • Protozoa (such as Cryptosporidium and Giardia)

For other filtration options, you can read about how to filter specific contaminants from water in our post titled Best Water Filter Systems For Your Home.

Final Thoughts

Most U.S. municipalities follow a similar water treatment method: collection, coagulation, flocculation, sedimentation, filtration, disinfection, storage, and distribution. Individual water treatment systems may customize the techniques, equipment, and chemicals used in these steps based on the water quality and the population’s needs. However, you must always remember that municipal water treatment doesn’t guarantee your water will be clean and safe to drink. Sometimes we need additional treatment systems, like reverse osmosis water filters or other home treatment solutions, to remove contaminants that weren’t eliminated at the plant or picked up as the water traveled to our homes.

But make no mistake. Municipal water treatment systems are some of the unsung heroes in our lives, and we must recognize the immense work and effort they put into serving our communities – which often goes unappreciated. Hopefully, after reading this post and learning about the complex behind-the-scenes processes that work to make our tap water safer and healthier, you’ll see how fragile our water is and why it’s essential to have clean, great-tasting water flowing through our taps.